1. Field of the Invention
The present invention relates to an air conditioning apparatus for an automobile, and in particular, to an air conditioning apparatus suitable for an automobile with no hot water source, such as an electric car.
2. Description of Related Art
In a conventional air conditioning apparatus for an automobile provided with an internal combustion engine, a provision is made for a cooling circuit of a refrigerant from a compressor driven by the engine and a heating circuit of hot water from an engine water jacket, which is independent from the cooling circuit. Namely, in a conventional type air conditioning apparatus for an automobile provided with an internal combustion engine, the cooler as an evaporator located in the refrigerating circuit for cooing the air and the heater located in the hot water circuit for heating the air are arranged separately in an air duct in which air is discharged to a cabin of the vehicle flows, so that the air flow is in contact with the air cooler and the air heater. Namely, the air is, at first, cooled by contacting the air cooler due to the heat exchange of the air with the refrigerant in the cooler, and is heated to a desired temperature by contacting the heater due to the heat exchange of the air with the hot water from the engine cooling system. Furthermore, the hot water heater is arranged in the air duct, so that a by-pass passageway is created that allows an amount of the air to by-pass the hot water heater, and an air mix damper is provided for controlling a ratio of the amount of air directed to the hot water heater and the amount of air directed to the by-pass passageway. The cooled air from the cooler after by-passing the hot water heater is combined with the air after contacting the hot water heater, so that a desired temperature of combined air is obtained in accordance with the degree of the opening of the air mix damper.
Such a type of air conditioning apparatus provided with a refrigerant cooler and a hot water heater cannot, however, be used for an electric car, due to the fact that the electric car does not have any heating source such as hot water. Thus, the electric car usually employs a single refrigerant line for a cooling operation and for a heating operation. Namely, in the electric car, the refrigerating cycle is constructed so as to obtain a refrigerating cycle for cooling and also for obtaining a "heat pump" for a heating operation. Namely, the refrigerating cycle is, as is well known, constructed by a compressor, an inside heat exchanger contacting the air flow in the duct, an outside heat exchanger contacting the outside air flow, and a pressure reduction device. During a cooling operation, the high pressure refrigerant from the compressor is, first, introduced into the outside heat exchanger where the refrigerant is condensed by emitting heat to the atmosphere, and the liquidized refrigerant is, after being subjected to pressure reduction at the pressure reduction device, introduced into the inner heat exchanger, where the refrigerant is evaporated thereby removing heat from the air flow in the duct and reducing the temperature of the air discharged to the cabin. During a heating operation, the high pressure refrigerant from the compressor is, first, introduced into the inside heat exchanger, where the refrigerant is condensed by emitting heat to the air flow in the duct and heating the air discharged to the cabin, and the liquidized refrigerant is, after being subjected to pressure reduction at the pressure reduction device, introduced into the outside heat exchanger, where the refrigerant is evaporated for removing heat from the outside air. It should be noted that temperature control of the air discharged from the duct into the cabin is effected by controlling the flow amount of refrigerant by controlling the rotational speed of the compressor.
Such an air condition system using the refrigerating cycle for both cooling and heating operations suffers from an intrinsic drawback caused by the fact that the same heat exchanger is switched between the cooling and the heating operations. Namely, during a switching from a cooling operation to a heating operation, the inner heat exchanger, which has previously worked as an evaporator (cooler), changes its function to a condenser (heater). Such a switching of the function of the inner heat exchanger from a cooler (evaporator) to a heater (condenser) causes the drops of water attached thereon during the previous cooling operation to evaporate, and the evaporated water together with the hot air heated by the heat exchanger is introduced into the cabin, so that the humidity in the cabin increases, causing condensation to form the windows. Furthermore, a relatively small cabin volume causes such condensation to occur very quickly after the said switching, which during movement of the vehicle, may reduce the field of vision of the driver. From a practical point of view, such a switching from a cooling operation to a heating operation is not usually required. However, during a heating operation, in addition to the inner heat exchanger working as a heater, an additional heat exchanger working as an evaporator for dehumidifying the air flow is often brought into operation. Thus, a switching from a usual heating operation to a dehumidifying operation, which frequently occurs, may also cause condensation to form on the windows.
In order to obviate the above-mentioned problem, the Japanese Un-Examined Patent Publication No. 1-239353 discloses a humidity sensor provided in the duct at a position adjacent to the inner heat exchanger. The detection of a humidity larger than a predetermined value causes the compressor to temporarily stop during a switching from a cooling or dehumidifying operation to a heating operation until the drops of water generated during the preceding operation are fully drained, however, this solution requires a certain amount of time to remove the moisture before the compressor is re-started after switching from the cooling operation to the heating operation. Furthermore, it may occur that the water drops make contact with the humidity sensor, thereby causing it to operate erroneously. In addition, the provision of a humidity sensor is not practical because of the increase in cost thereof.